Generally, rendering addresses the processing of image data pertaining to an object for purposes of displaying the rendered object to a viewer such as human visual processing. When the display of the rendered object is via a printed rendering, properties of the printer may limit the gradations in tone and continuity of the object as expressed in pixels. A means of achieving apparent tonal graduations is via the application of halftoning. Halftoning may be described as applying a high frequency/low frequency dichotomy having a low frequency attribute and a high frequency attribute. For example, the low frequency attribute may be a local area of the output image designated as a halftone cell. Each equal-sized cell relates in size and location to a corresponding area of the continuous-tone input image. Within each cell, the high frequency attribute may be a centered, variable-sized, halftone dot or pixel composed of ink or toner.
Single level digital halftoning uses a raster image or bitmap within which each monochrome picture element, or pixel, may be on or off as to the application of a colorant, that is, tone or no tone, ink or no ink, may be applied. For a particular color, including black, the ratio of the colored area to the non-colored area of the output cell corresponds to the luminance or gray level of the input cell. From a suitable distance, human visual processing averages both the high frequency apparent gray level approximated by the ratio within the cell and the low frequency apparent changes in gray level between adjacent equally-spaced cells and centered dots. To achieve this effect, the digital halftone cell contains groups of monochrome pixels within the same-sized cell area. A higher screen frequency halftone may be applied to text quality, for example, via an eight-by-eight sized cluster dot screen. But such halftoning does not maximize output pixel coverage and so may result in broken shapes and missing lines, particularly for fine rendering of fonts and lines of light gray levels. There remains a need for improved rendering quality for color multi-function-printers.
Printing devices that are capable of printing objects rendered in pixels having two or more density levels, that is two or more gray levels, have a second means of approaching the appearance of a continuous tone scale over the binary state afforded single level halftoning. Accordingly, multilevel halftoning, i.e., the application of two or more density levels for toning or pigmenting may be applied in conjunction with halftoning to generate refined renderings.